1. Field of the Invention
The invention relates to rotary drill bits for drilling wellbores in subsurface formations and, more particularly, to drill bits that can be left in the wellbore and drilled through, if needed to do so.
2. Description of Related Art
In the drilling of wellbores in subsurface formations it is common place to connect the drill bit to relatively heavy and thick-walled drill pipe that is rotated at the earth's surface by a rotary table or rotated within the wellbore by a downhole motor or turbine. When a particular section of the wellbore has been drilled, the drill pipe and the drill bit are recovered to the surface and a string of relatively thin-walled pipe, called casing, is set within the wellbore and cemented in place.
When drilling into relatively high pressure zones problems can occur when trying to reenter the wellbore with the casing string. Oftentimes in drilling in the Offshore Gulf Coast of the U.S.A., when the drill bit and the drill string breach a high pressure zone, the drilling operation may be ceased so that the operator can pump heavy drilling fluid into the wellbore to prevent the high-pressure fluids from the high pressure formation from rapidly rising to the surface. When the drilling has ceased, the wellbore tends to swell or slough or collapse, and removal of the drill string and the drill bit can be difficult and sometimes impossible. If the drill string can be removed, the reentry into the wellbore with the casing string can be very difficult and sometimes impossible, for the above reasons. All of these problems cause the drilling operation to be delayed, with hundreds of thousands of dollars in increased costs. Worst of all, if the drill bit cannot be removed then the wellbore must be abandoned or side-tracked.
There is a need for methods and related equipment that will allow the operator to drill through the high pressure zones, leave the drill bit in place if needed, and eliminate the need for a drill pipe removal and casing string tripping operation.